Intracellular catabolism of radioiodinated proteins can be studied by assay of trichloroacetic acid-soluble radioactivity either in isolated phagolysosomes after intravenous injection into animals, in tissue slices from these animals, or by addition of the labelled proteins to particulate or soluble tissue preparations. Structural integrity of isolated phagolysosomes can be studied by measuring osmotic release of labelled protein. Studies will be performed to establish whether the stimulation of intralysosomal proteolysis of radioiodinated albumin in isolated phagolysosomes by ATP is due to maintenance of intralysosomal acidity, and attempts will be made to determine the mechanisms involved. The effects of substances known to interfere with lysosome function will be tested in the system. Studies will be performed to determine if fusion of phagosomes with lysosomes will occur in tissue slices or particulate suspensions by assaying the capacity for proteolysis. If these attempts are successful, studies will be performed to determine energy requirements and the effects of various metabolic inhibitors and agents known to interact with microtubules. Experiments will be performed to determine the nature of the inhibitory effect of certain agents on proteolysis in tissue slices. The nature of the particles involved in the catabolism of radioiodinated cytochrome C' in mouse kidney and liver preparations will be determined. Enzymes involved will be purified, and substrate specificity and other properties of the enzymes will be studied. BIBLIOGRAPHIC REFERENCE: John L. Mego, Phagolysosome Reconstitution. Proteolysis in Liposomes Containing Pronase or Lysosomal Enzymes, J. RE Soc. 20:27a (1976).